PREPARATION OF STANDARD REFERENCE SAMPLES

All of the SRSs used in this evaluation were prepared by USGS personnel located in Lakewood, Colorado, and were analyzed for analyte concentrations and physical property values before mailing. A supply of these SRSs is maintained and is available to purchase by participating laboratories and USGS offices for use in their quality-control programs.

Trace constituents sample T-179 was prepared using water collected from Coal Creek southwest of Boulder, Colorado. The water was pumped through a 0.2- and 0.1-micrometer (mm) filter into a 1325-liter (L) polypropylene drum. The water was continuously circulated and passed through a 0.1-mm filter and ultraviolet sterilizer for 24 hours. The water was then acidified to a pH <2 with nitric acid (HNO3) and chlorinated to 5 parts per million (ppm) free chlorine with sodium hypochlorite. Some trace constituent concentrations were adjusted by adding reagent-grade chemicals. The sample was circulated through a 0.1-mm filter and an ultraviolet sterilizer for an additional 24 hours prior and during bottling. The 500-milliliter (mL) polypropylene bottles and caps were acid leached with 0.16N HNO3, deionized-water rinsed, and autoclave sterilized.

Major constituents sample M-172 was prepared using water collected from Chicago Creek near Idaho Springs, Colorado. The water was pumped through a 0.2- and 0.1-mm filter into a 1325-L polypropylene drum. The water was continuously circulated and passed through a 0.1-mm filter and ultraviolet sterilizer for 24 hours. The water was then chlorinated to 5-ppm free chlorine with sodium hypochlorite. Some major constituent concentrations were adjusted by adding reagent-grade chemicals. The sample was circulated an additional 24 hours, then allowed to sit for 48 hours. During bottling, the sample was pumped through an ultraviolet sterilizer and a 0.1-mm filter. The 500-mL polypropylene bottles and caps were acid leached with 0.16N HNO3, deionized-water rinsed, and autoclave sterilized.

Nutrient constituents sample N-83 was prepared in a 50-L polypropylene carboy using deionized water. This SRS was prepared a couple weeks prior to sample distribution. The water was circulated through a 0.1-mm filter and kept chilled with ice (12 degrees Celsius) during the entire preparation procedure. Ultraviolet sterilization was performed until the addition of reagent-grade chemicals. The 60-mL amber glass vials and teflon-faced rubber-lined caps were acid leached with 0.1N hydrochloric acid (HCl), deionized-water rinsed, and autoclave sterilized.

Nutrient constituents sample N-84 was prepared in a 190-L polypropylene drum using water collected from South Clear Creek south of Georgetown, Colorado. This SRS was prepared a couple weeks prior to sample distribution. The water was circulated through a 0.1-mm filter and kept chilled with ice (12 degrees Celsius) during the entire preparation procedure. Ultraviolet sterilization was performed until the addition of reagent-grade chemicals. The 250-mL polyethylene bottles were acid leached with 0.1N HCl, deionized-water rinsed, and autoclave sterilized.

Low ionic-strength constituents sample P-43 was prepared in a 600-L polypropylene drum with snow collected from Silverthorne, Colorado. The desired phosphate and fluoride concentrations were obtained by adding reagent-grade chemicals. Prior and during bottling, the sample was circulated through a 0.1-mm filter and an ultraviolet sterilizer. The 500-mL polypropylene bottles and caps were acid leached with 0.16N HNO3, deionized-water rinsed, and autoclave sterilized.

Mercury sample Hg-39 was prepared in a 45-L glass carboy using water collected from Chicago Creek near Idaho Springs, Colorado. It was preserved with 5 mL/L of 12 N HCl. The desired mercury concentration was obtained by adding a mercury standard solution. The 250-mL borsilicate glass bottles and teflon-lined caps were new, acid leached, and deionized-water rinsed.